EP3540486B1

EP3540486B1 - Compact multichannel optical rotary joint

Info

Publication number: EP3540486B1
Application number: EP18162195.4A
Authority: EP
Inventors: Gregor Popp; Jacques Abenhaim
Original assignee: Schleifring GmbH
Current assignee: Schleifring GmbH
Priority date: 2018-03-16
Filing date: 2018-03-16
Publication date: 2020-06-24
Anticipated expiration: 2038-03-16
Also published as: CN111328379A; US10962722B2; US20210063648A1; WO2019175206A1; EP3540486A1; CN111328379B

Description

Field of the invention

The invention relates to Multi-channel rotary joints for optical signals. Such rotary joints a capable of simultaneously coupling a plurality of optical signals between two devices which are rotatable against each other.

Description of the related art

Optical rotary joints for simultaneous coupling of a plurality of optical signals between two units which are rotatable against each other, preferably have a derotating element. Such a derotating element may be a Dove prism which is rotated with half the angular speed of the relative rotation of the two units. On both sides of the Dove prism, collimators are provided for beam forming. The collimators provide a parallel beam of light from the light coupled by an optical fiber, and provide an increased beam diameter which is significantly larger than the fiber diameter.
WO 2007/010362 A2 , US 2007/0184934 A1 , US 5,371,814 and US 5,271,076 disclose such a multi-channel fiber optic rotary joint, where a Dove prism is held rotatably within an outer housing. The design is optimized for a shortest-possible optical path between the two collimators, such that mechanical tolerances, specifically in the alignment of the collimators and the Dove prism, have the smallest-possible effect on signal transmission or attenuation. Therefore, the collimators are mounted as close as possible on both sides of the Dove prism.
US 5,157,745 discloses a further multi-channel fiber optic rotary joint. Here, cylindrical lenses and lateral adjustors are provided to compensate for mechanical tolerances of the collimators and the Dove prism.
US 5,176,331 discloses a rotation compensation device for a cable drum.
US 5,442,721 discloses a comparatively short rotary joint using all lens collimators. A very important aspect of these rotary joints is their comparatively short optical path which helps to minimize the adverse effects of angular deviation of the optical paths and therefore reduces coupling losses.
The optical fibers attached to the collimators typically leave the housings parallel to the rotation axis. There are alternative designs where the fibers are bent by 90 degrees to leave the housing perpendicular/orthogonal to the rotational axis. Since optical fibers are available that allow a tight bending radius of e.g. 2.5mm at tolerable attenuation increase and life time reduction this is a solution allowing short collimators with small dimension in axial direction. Other implementations are disclosed in US7876985B2 .

Summary of the invention

The problem to be solved by the invention is to provide a multi-channel rotary joint which has a comparatively small outer diameter. It should have comparable or even lower coupling losses than the rotary joints known from prior art. Therefore, the rotary joint should provide a high mechanical and optical precision and therefore comparatively low mechanical tolerances. Manufacturing and assembly should be easy and simple, keeping the overall costs low. Furthermore, the design should be usable for high rotational speeds.
Solutions of the problem are described in the independent claims. The dependent claims relate to further improvements of the invention.
In a first embodiment, an optical rotary joint comprises a housing, a hollow shaft, a bevel gear, a derotating element like a prism, and two collimators. The rotary joint has a center axis. The housing comprises two housing sections which are separated by a gap in an axial direction. The gap is large enough to hold the bevel gear, as will be shown later. The housing sections are rotatable against each other around the center axis. The first housing section has in axial direction a first inner side and a first outer side. The second housing section has in axial direction a second inner side and a second outer side. The first inner side of the first housing section is oriented towards the second inner side of the second housing section. The outer sides of the housings preferably bear collimators, such that preferably a first collimator is arranged at the first outer side of the first housing section, and a second collimator is arranged at the second outer side of the second housing section.
The hollow shaft is arranged essentially within the housing sections and aligned with the center axis. The hollow shaft has a first end which extends into the first housing section, and a second end which extends into the second housing section. The first housing section is supported on the first end of the shaft by at least one bearing, preferably by two, three or four bearings. The second housing section is supported on the second end of the shaft by at least one bearing, preferably by two, three or four bearings. Furthermore, the first housing may in addition be supported by at least one bearing, preferably by two, three or four bearings on the second end of the shaft. The hollow shaft further has a prism holder between the first and the second end. The prism holder is further preferably located within the first housing section.
The bevel gear is located in the gap between the first housing section and the second housing section. It comprises three wheels. A first wheel is at the first inner side of the first housing section. A second wheel is at the second inner side of the second housing section, At least one third wheel is arranged, such that it meshes with the first wheel and the second wheel. The first and second wheels are coaxial to the center axis and opposing to each other, such that the first wheel opposes to the second wheel. Preferably, the first wheel and the second wheel have the same diameter, have preferably the same size, and most preferably are identical to each other. The at least one third wheel has an axis which is oriented radially to the center axis. The axis is fixedly attached to the hollow shaft or is one part with the hollow shaft. Therefore, when the housing sections are rotated against each other, causing the first and the second wheel to rotate, the third wheel will also rotate and generate a rotation of the hollow shaft around the center axis. The rotational speed of the hollow shaft is precisely half the speed of the housing sections relative to each other. Furthermore, the bevel gear is displaced in an axial direction from the prism holder, such that the axis of the third wheel in connection with the hollow shaft is comparatively short, and the third wheel is comparatively close to the hollow shaft. This allows to keep the outer diameter of the rotary joint comparatively small. Actually, it may be build such that the Dove prism, which is located in the prism holder outer the hollow shaft, is the largest component and defines the outer diameter of the whole rotary joint. For best precision, at least one bearing is on each side of the prism holder. Furthermore, it is preferred to have at least one bearing on each side of the bevel gear. Furthermore, preferably two bearings are within each housing section.
As the prism in the prism holder and the bevel gear are arranged along the hollow shaft and therefore are separated in an axial direction, the outer diameter of the rotary joint can be minimized to the diameter of the largest component, which usually is the Dove prism. Furthermore, this design is usable to very high rotational speeds, as the diameter is very small and therefore the centrifugal forces are low. As the main extension of the rotary joint is in axial direction along the center axis, the distances between the bearings are comparatively large, which allows for a stable support of the components without having the risk of tilting or oscillations during operation.
The bevel gear may comprise only one third wheel, but it is preferred to have at least one two or a higher number, like three or four or more third wheels. Preferably, the third wheels are arranged equidistant around the hollow shaft. This evenly distributes the forces to the hollow shaft and therefore increases precision. To avoid oscillation at high rotational speeds, it may be desirable to have slightly different distances between the third wheels.
To have a compact assembly, it is preferred if the third wheel has a smaller diameter than the first and the second wheel. For a compact assembly, the third wheel may also have a simplified bearing, like a friction bearing or a plane bearing. For high rotational speeds, it may also have a ball bearing.
Preferably, the housing has a cylindrical outer contour, and most preferably the first housing section has the same outer diameter as the second housing section. It is further preferred to provide a cover on the gap which may be connected either the first or the second housing section and rotate freely with respect to the other housing section.
Preferably, the prism holder is a section of the hollow shaft for holding and/or accommodating the prism. The prism holder may also be part of the hollow shaft itself. It may also be a separate part held by the hollow shaft. Such a separate part simplifies assembly. It may also allow to adjust the prism within the holder before assembly with the hollow shaft. If the prism holder is a part of the shaft, then the prism may be mounted directly (e.g. by gluing) into the shaft.
Preferably, the prism holder has an outer diameter larger than the outer diameter of the first end of the hollow shaft and the second end of the hollow shaft. Normally, the usable cross-section of a Dove prism is less than the height and the width of the Dove prism. Furthermore, some material is needed for the prism holder to hold the prism precisely at a predetermined position with respect to the center axis. An inner bore of the hollow shaft is required with such a diameter that all the collimated beams from the collimator may be guided between the collimators and the Dove prism. Due to the smaller usable cross-section of the Dove prism, the total area of the beams is significantly smaller than the height and the width of the Dove prism, and it is further significantly smaller than the prism holder. As the bearings between the hollow shaft and the housing sections are on the first end and second end of the hollow shaft, but not on the prism holder, these bearings have an inner diameter which is preferably smaller than the outer diameter of the prism holder.
In a further embodiment, the bevel gear is a crown gear which preferably uses crown wheels. Such a crown gear is a specific modification of a bevel gear, where the wheels have an angle of 90 degrees with respect to their axis, and the teeth of the wheels are basically directed parallel to the axis.
A method for adjusting the bevel gear of an optical rotary joint, for example as mentioned above, comprises a first step in which the whole unit is assembled by using means having a surface coating. The surface coating may have a predetermined coating thickness, preferably in the range between 1 µm and 30 µm. In a second step, the unit is disassembled and uncoated wheels or wheels with a thinner coating are inserted. Now, there is some mechanical play between the wheels due to the thickness difference the first set of wheels having a thicker coating and the second set of wheels having a thinner coating. This play allows for lower movement forces and for better rotation of the wheels.

Description of Drawings

In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings.

Figure 1: shows a sectional view of a first embodiment.
Figure 2: shows an outer view of the rotary joint.
Figure 3: shows a further outer view.
Figure 4: shows an embodiment with removed cover.
Figure 5: shows details of the gear.

In Figure 1, a sectional view of a first embodiment is shown. The optical rotary joint 100 comprises a housing 200, a hollow shaft 300, a bevel gear 400, a derotating element like a prism 500, and two collimators 610, 620. The rotary joint has a center axis 101 which is also the rotation axis.
A first collimator 610 having first optical fibers 611 at a first side of the rotary joint and opposing thereto at a second side of the rotary joint a second collimator 620 having second optical fibers 621 define an optical path. Light coming from first optical fibers is collimated by a first collimator such that a parallel beam of light preferably having a larger diameter than the core diameter of the optical fiber is generated. This beam of light is guided through the hollow shaft 300 and the Dove prism 500 located therein. Here, for simplicity the term of a Dove prism is used. Of course, any other prism suitable for derotation or any other derotating element may be used. The beam of light is collected by the second collimator 620 and coupled into a second optical fiber 621. There may be one optical path from a first optical fiber to a second optical fiber or a plurality of optical paths from the plurality of first optical fibers to a plurality of second optical fibers. Basically, there may be any number of such optical paths. It is obvious that light may be coupled from the first side to the second side or vice versa from the second side to the first side. There may also be mixed directions of optical paths, like a first path from the first side to the second side and a second path from the second side to the first side.
The first collimator 610 is mounted to a first housing section 210 and the second collimator 620 is mounted to a second housing section 220. The first section 210 and the second housing section 220 are separated by a gap in an axial direction. The gap holds the bevel gear 400. The housing sections are independently rotatable against each other around the center axis. The first housing section has in an axial direction a first inner side 211 and a first outer side 212. The second housing section has in an axial direction a second inner side 221 and a second outer side 222. The first inner side 211 of the first housing section 210 is oriented towards the second inner side 221 of the second housing section 220.
The hollow shaft 300 is arranged essentially within the first housing section 210 and the second housing section 220. It is further aligned with the center axis 101. The hollow shaft 300 has a first end 310 which extends into the first housing section 210 and a second end 320 which extends into the second housing section 220. The first housing section is supported on the first end of the shaft by at least a first bearing 350, and the second housing section is supported on the second end of the shaft by at least a second bearing 360. The at least one first bearing comprises preferably two, three or four bearings, or most preferably two first ball bearings 351, 352.
Preferably, a first outer ball bearing 351 is close to the first outer side 212 and a first inner ball bearing 352 is close to the first inner side 211, wherein the prism holder and/or the prism is between the first outer ball bearing 351 and the first inner ball bearing 352. Most preferably, the bearings 351, 352 are axially distant from the prism holder and/or the prism. This results in a comparatively high axial stability, further increasing coupling losses. As the bearings are axially distant from the prism, comparatively small and therefore precise bearings may be used, as the bearings must no more enclose the prism.
The at least one second bearing comprises preferably two, three or four bearings, or most preferably two second ball bearings 361, 362.
The hollow shaft further has a prism holder 330 between the first and the second end. The prism holder is further preferably located within the first housing section 210.
The bevel gear 400 is located in the gap 230 between the first housing section 210 and the second housing section 220. The gear comprises at least three wheels. A first wheel 410 is at the first inner side 211 of first housing section 210. A second wheel 420 is at the second inner side 221 of the second housing section 220. At least one third wheel 430 is arranged such that it meshes with first wheel 410 and the second wheel 420. The first wheel 410 and the second wheel 420 are coaxial to the center axis 101 and oppose each other such that the first wheel opposes the second wheel. It is preferred, if the first wheel and the second wheel have the same size and most preferably they are identical to each other. The at least one third wheel 430 has an axis 450 which is oriented radially to the center axis. The axis 450 is fixedly attached to the hollow shaft 300 or is one part with the hollow shaft. It may be a bolt or a screw. Therefore, rotation of the housing sections against each other causing the first or second wheel to rotate will also cause the third wheel to rotate and to generate a rotation of the hollow shaft around the center axis. The rotational speed of the hollow shaft is precisely half the speed of the housing sections relative to each other. As can be seen here, the bevel gear is displaced in axial direction from the prism holder, such that the axis of the third wheel in connection with the hollow shaft is comparatively short and the third wheel is comparatively close to the hollow shaft. By this arrangement the outer diameter of the rotary joint is comparatively small. This also keeps the rotating masses and the radius of the rotating masses small, such that the rotary joint can easily follow quick accelerations. This further reduces forces to the rotary joint components and therefore maintains a longer lifetime keeping the high precision of the rotary joint. Tests have shown that this design provides a long term stable precision and therefore maintains lower transmission losses for longer periods of time. Furthermore, as due to the comparatively large length of the rotary joint, the bearings of the housing section, specifically the housing section within which the Dove prism is located, can have large distances which further ensures a large angular stability and therefore high precision of the rotary joint. The bevel gear may comprise a plurality of wheels. In this embodiment, two wheels opposing to each other are shown.
The prism holder 330 holds the prism 500 within the hollow shaft 300. Of course, the prism may also be mounted directly into the hollow shaft, but a prism holder preferably allows some adjustment of the prism.
In Figure 2, an outer view of the rotary joint with the first housing section 210 and the second housing section 220 is shown. There may be a cover 240 covering the gap and the bevel gear therein.
In Figure 3, an outer view further indicating the center axis 110 and the rotations of the first housing section 210 as well as the rotation of the second housing section 220 is shown.
Figure 4 shows the embodiment of the previous figure, but with removed cover 240. It shows the bevel gear which is a special bevel gear embodiment, called a crown gear. In a crown gear, the pitch cone angle is 90 degrees. Further details are explained in the next figure.
Figure 5 shows details of the bevel gear (crown gear). The bevel gear 400 is located in the gap 230 between the first housing section 210 and the second housing section 220. The gear comprises at least three wheels. A first wheel 410 is at the first inner side 211 of first housing section 210. A second wheel 420 is at the second inner side 221 of the second housing section 220. At least one third wheel 430 is arranged such that it meshes with first wheel 410 and the second wheel 420. The first wheel 410 and the second wheel 420 are coaxial to the center axis 101 and oppose each other such that the first wheel opposes the second wheel. Here, if the first wheel and the second wheel have the same size and most preferably they are identical to each other. This figure shows two third wheels 430, one to the front and one opposing thereto, to the rear in the figure. Each third wheel 430 has an axis 450 which is oriented radially to the center axis.
The axis 450 is fixedly attached to the hollow shaft 300 or is one part with the hollow shaft. It may be a bolt or a screw. Therefore, rotation of the housing sections against each other causing the first or second wheel to rotate will also cause the third wheel to rotate and to generate a rotation of the hollow shaft around the center axis.

List of reference numerals

100: optical rotary joint
101: center axis
110: rotation of first housing section
120: rotation of second housing section
200: housing
210: first housing section
211: first inner side
212: first outer side
220: second housing section
221: second inner side
222: second outer side
230: gap
240: cover
300: hollow shaft
310: first end
320: second end
330: prism holder
350: first bearing
351, 352: first ball bearings
360: second bearing
361, 362: second ball bearings
400: bevel gear
410: first wheel
420: second wheel
430: third wheels
450: axis of third wheels
500: prism
610: first collimator
611: first optical fibers
620: second collimator
621: second optical fibers

Claims

Optical rotary joint (100) comprising a housing (200), a hollow shaft (300), a bevel gear (400), a prism (500), and two collimators (610, 620) the rotary joint (100) having a center axis (101),
the housing (200) comprising a first housing section (210) and a second housing section (220) separated by a gap (230) in an axial direction from each other,
the first housing section (210) and the second housing section (220) being rotatable against each other and around the center axis (101),
the first housing section (210) having in axial direction a first inner side (211) and a first outer side (212),
the second housing section (220) having in axial direction a second inner side (221) and a second outer side (222),
the first inner side (211) being oriented towards the second inner side (221),
a first collimator (610) is arranged at the first outer side (212) and a second collimator (620) is arranged at the second outer side (222),
the hollow shaft (300) being aligned with the center axis (101),
the hollow shaft (300) having a first end (310) extending into the first housing section (210) and a second end (320) extending into the second housing section (220),
the first end (310) of the hollow shaft (300) is support for at least a first bearing (350), the first bearing (350) further being support of the first housing section (210) and
the second end (320) of the hollow shaft (300) is support for at least a second bearing (360), the second bearing (360) further being support of the second housing section (220),
the hollow shaft (300) having a prism holder (330) between the first end (310) and the second end (320), the prism holder (330) is located within the first housing section (210),
the bevel gear (400) being located in the gap (230) between the first housing section (210) and the second housing section (220),
the bevel gear (400) comprising a first wheel (410), a second wheel (420) and at least one third wheel (430),
the first wheel (410) being coaxial to the center axis (101) at the first inner side (211) of the first housing section (210),
the second wheel (420) being coaxial to the center axis (101) and opposing to the first wheel (410) at the second inner side (221) of the second housing section (220) and having the same diameter as the first wheel (410), the at least one third wheel (430) being between the first wheel (410) and the second wheel (420) and in mesh with the first wheel (410) and the second wheel (420),
the at least one third wheel (430) having an axis (450) oriented radially to the center axis (101) and being fixedly attached to or being one part with the hollow shaft (300), such that a rotation of the first housing section (210) against the second housing section (220) with a first angular speed results in a rotation of the hollow shaft (300) with half of the first angular speed,
the bevel gear (400) being displaced in axial direction from the prism holder (330),
the prism (500) being located in the prism holder (330) of the hollow shaft (300).
Optical rotary joint (100) according to claim 1,
characterized in, by
the gear (400) being a crown gear.
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the gear (400) comprising 2, 3 or 4 third wheels (430), preferably arranged equidistant around the hollow shaft (300).
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the at least one third wheel (430) having a smaller diameter than the first wheel (410).
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the at least one third wheel (430) having a friction bearing or plain bearing.
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the housing (200) having a cylindrical shape.
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the first housing section (210) having the same outer diameter as the second housing section (220).
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the housing (200) further comprising a cover (240) on the gap (230).
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the prism (500) being a dove prism or an Abbe-Koenig prism.
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the prism holder (330) having a larger outer diameter than the outer diameter of the first end (310) and of the second end (320) of the hollow shaft (300).
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
at least one of the first bearing (350) and second bearing (360) comprising a ball bearing.
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the first bearing (350) and the second bearing (360) being displaced in axial direction from the prism holder (330).
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
the inner diameter of the first bearing (350) and the inner diameter of the second bearing (360) being smaller than an outer diameter of the prism holder (330).
Optical rotary joint (100) according to any one of the preceding claims, characterized in, by
at least one bearing (350, 360) being at each side of the prism holder (330).